1. Field of the Invention
This invention relates to thin film materials, such as paraelectric Ba1xe2x88x92xSrxTiO3 (BST) based thin films, and methods for producing such materials.
2. Discussion of the Related Art
Thin films of dielectric materials (ferroelectric and paraelectric materials) have applications in a variety of microelectronic devices. Ferroelectric and paraelectric materials, such as barium strontium titanate, Ba1xe2x88x92xSrxTiO3 (BST), Pb(Zr,Ti)O3 (PZT), SrTiO3 (STO), SBT (SrBi2Ta2O3), and tantalum oxide (Ta2O5), thin films are commonly used in memory devices, such as ultra large scale integrated DRAMs/FRAMs, dielectric capacitors, electro-optic, piezoelectric and pyroelectric devices. Additionally, such films are suitable for applications in a large family of tunable and electronically controllable microwave devices. Such devices include voltage tunable phase shifters, capacitors, oscillators, filters, delay lines, resonators, and parametric amplifiers.
In particular, the thin film technology, such as a BST-based film, allows high frequency device operation (about 1 to about 100 GHz) with minimum power consumption (e.g., less than about 10 volts), low noise, integration compatibility with other semiconductor microwave devices. BST-based thin film technology additionally permits realization of compact, light weight, integratable, conformable, and tunable microwave devices to be realized.
However, because conventional BST film processing techniques include treatment temperatures often greater than about 700xc2x0 C., interdiffusion is caused at the interface of the film and the substrate. Such interdiffusion results in undesirable interfacial phases that alter the dielectric properties of the film. As the film becomes multi-phased, the dielectric properties of the ferroelectric film degrade due to the addition of one or more non-ferroelectric phases. In microwave applications, this translates to an undesirable power loss in the device. Such elevated temperatures are often generated during deposition of the film upon the substrate, post-depositing annealing and in situ deposition substrate heating. If paraelectric BST-based thin film technology is to be efficiently integrated with other microwave components, this processing temperature must be lowered because at conventional treating temperatures, e.g., generally greater than about 700xc2x0 C., the above-described interdiffusion occurs between the film and the semiconductor substrate.
Conventional BST-based thin films deposited onto a semiconductor substrate are traditionally processed at temperatures in excess of 750 xc2x0 C. As a result, the paraelectric BST-based thin film-substrate interface is subjected to high temperature heat treatment, for example up to about 1200xc2x0 C. Thus, a structurally and chemically abrupt film-substrate interface is created, however interfacial phase formation results due to film-substrate elemental or chemical diffusion.
Techniques have been developed to eliminate the interfacial phases including
(1) low temperature annealing (TA) or lower substrate temperatures (TS) during film deposition;
(2) using refractory ceramic substrates, such as, MgO, LaAlO3, SrTiO3, sapphire, and glass;
(3) using thermally stable microwave friendly barrier layers sandwiched between the semiconductor substrate and the BST-based thin film; and
(4) low-temperature deposition techniques such as thermal metalorganic chemical vapor deposition (MOCVD).
Lowering of TA or Ts is counter productive because the dielectric, insulating, and tunability properties, as well as the reliability of paraelectric thin film materials, are strongly influenced by film crystallinity, film stress, quality of the film-substrate interface, and the film microstructure, each of which are a function of TA and TS. Adequate post-deposition annealing or in situ substrate heating is required to impart crystallinity, increase the overall grain size of the-film, and to remove film strain by filling oxygen vacancies. These factors are particularly important since microwave dielectric loss in BST-based thin films is strongly influenced by stoichiometric deficiencies, which create vacancies, film strain, and the presence of a large grain boundary to grain ratio. Therefore, in order to reduce the microwave dielectric loss, the BST-based films must be fully crystallized or developed. However, a fully crystallized or developed thin film with optimal dielectric and insulating properties cannot be achieved by simply lowering the processing temperatures (TA and/or TS).
The technique of using a refractory ceramic substrate allows the film dielectric properties to be optimized via high temperature processing. However, these film-ceramic substrate structures are not directly integratable with semiconductor components or devices, that is, non-semiconductor substrates offer incompatibility with other semiconductor-based devices, posing integration issues. These film-ceramic substrate structures are essentially discrete components and are not directly integratable with other microwave semiconductor-based microwave devices via traditional direct growth and deposition techniques. The use of microwave friendly barrier layers, such as single and binary oxides and superconductors, permits the use of low cost non-microwave friendly Si substrates and higher processing temperatures. However, barrier layers add extra processing steps to the device fabrication process. Additional processing steps are undesirable from the device fabrication, yield, reliability and cost perspectives.
Thin films of, for example, BST-based materials, on MgO substrates have been achieved at temperatures as low as 600xc2x0 C. via thermal metalorganic chemical vapor deposition (MOCVD). The processing temperature for the MOCVD technique is 200xc2x0 C. lower than for other BST film deposition techniques. However the MOCVD growth method has difficulty maintaining film stoichiometry due to reaction of the precursor elements in the vapor phase prior to deposition on the substrate. Precise stoichiometric control of the film is necessary to obtain the film composition influenced dielectric and insulating properties.
The invention relates to a method for producing paraelectric thin film materials which avoids undesirable film-substrate interdiffusion and formation of undesirable interfacial phases, while providing high quality film crystallinity and a fully developed thin film microstructure, required to achieve the desired microwave frequency dielectric and insulating properties. Specifically, deposition of a ferroelectric film on a SiC substrate obviates interfacial phase formation. This result is achieved even with treatment temperatures greater than 700xc2x0 C., which temperatures are necessary to fully crystallize and develop the film. This method achieves the result even with temperatures greater than 800xc2x0 C.
Because the material resulting from the method of the invention utilizes semiconductor substrates, the material may be directly integrated with other semiconductor components, such as those used in microwave devices.
The present invention includes deposition of a paraelectric film on a refractory, high temperature, and thermally stable semiconductor substrate, comprising for example, silicon carbide (SiC). SiC is a semiconductor material used for high temperature, high power and high frequency device applications. It is believed that the large bond energy of SiC is the cause of its thermal stability at high temperatures, and this thermal stability at high temperatures impedes the thermal processing related interdiffusion and undesirable interface phase formation between the paraelectric thin film and the substrate. The paraelectric film may be, for example, a BST-based film.
The method of the invention preferably uses physical vapor deposition (PVD) methods, such as pulsed laser deposition (PLD) and sputter deposition methods. Alternatively, the method of the present invention can employ chemical deposition methods such as metalorganic solution deposition (MOSD), dip coating, and chemical vapor deposition (CVD) techniques.
For PVD deposition of paraelectric films, such as BST-based films, the substrate may be heated to temperatures (TS), typically greater than about 750xc2x0 C., sufficient to achieve the fully crystallized film required to obtain the optimized microwave dielectric and insulating properties. Since the SiC substrate is a strongly bonded, high temperature material, there is no formation of undesirable film-substrate interfacial phases at the interface of the BST-based film and the SiC substrate after thermal treatment. The same is true for deposition techniques that require a post-deposition high temperature annealing treatment (generally in the range of about 700xc2x0 C. to 1100xc2x0 C.), such as the metalorganic solution deposition.
Operation under velocity saturation conditions permits high DC and RF currents to develop and permits efficient RF operation throughout the microwave frequency region. The high breakdown voltage of SiC permits high drain bias voltages to be applied, which are necessary to obtain high RF output power. Theoretical analysis predicts that SiC devices have a microwave power capability at room temperature that is approximately a factor of four greater than comparable devices fabricated from GaAs or Si.
The integration of paraelectric BST-based thin film with SiC substrates and epilayers is not only important for providing thermal stability between the BST-based film and SiC substrate at high processing temperatures, but is also of desirable for the integration of voltage tunable low cost, BST-based thin films with other SiC based microwave devices. Such applications include microwave power amplifiers that can be used, for example, in phased array radars, base station transmitters for mobile communications, and high efficiency and broadband radar transmitters. Thus, if the power circuitry of radar and communications systems is SiC based, then a paraelectric thin film phase shifter material deposited on a SiC substrate simplifies component integration issues.
Due to its wide bandgap (which permits operation at elevated temperatures), high bond strength (excellent thermal stability), high thermal conductivity (which permits higher power density), high electric field strength (large breakdown field which permits higher operating voltage), and its high electron saturation velocity (which permits high operating current), SiC is desirable for high temperature, high power and high frequency (microwave) device applications. SiC is well suited for microwave devices because of its low dielectric constant (4Hxe2x80x94SiC=10.0), low dielectric loss ( less than 1.0%), high thermal conductivity (4.0 W/xc2x0 K-cm), high saturation velocity (2.0xc3x97107 cm/s), and large breakdown field strength (3.5xc3x97106 V/cm). Specifically, the low dielectric constant produces reduced device impedances. Thus, the same device impedance, a larger device area can be used which in turn permits high RF power levels to be developed. The DC and RF device performance of high power microwave devices depends upon the ability to extract heat due to dissipated power. Hence, a high thermal conductivity is desirable. The DC and RF currents that flow through a microwave device are directly dependent on the charge carrier velocity versus electric field transport characteristics of a semiconductor material, and therefore a high saturation velocity is desirable. The magnitude of the electric field that produces saturated charge velocity is also important because the device must be able to develop the saturation field to obtain maximum RF performance and high frequency operation. The saturation fields for 4H and 6H SiC are Esxcx9c60 kV/cm and Esxcx9c200 kV/cm, respectively.